qml: Make ownership of CompiledUnit more clear

The coverity scan utility didn't understand the code flow and assumed
the compiledData would be leaked. Use a QScopedPointer and have the
::backendCompileStep() forward the ownership. From what I see the code
has not leaked memory.

Fixes: CID 10605, CID 10607

Change-Id: I7759f681871bbe12e2aa320a5f39c47c70f4e4e0
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>